近百年长江口盐沼发育对流域来水来沙的响应

Responses of the developments of estuary and coastal areas to both the environmental changes and human activities in the drainage basins of large rivers are one of the core contents of recent researches of several international scientific organizations (e.g., IGBP, LOICZ, IAG). The impacts of both the Three Gorges Dam Project and the South-to-North Water Diversion Project on the geomorphological pattern of the Yangtze estuary in the future, have become the focus problem that needs to be addressed by the scientific community of estuary and coastal researches. Continuous field survey of geomorphology and hydrology is an effective alternative for understanding the recent geomorphological dynamic of the Yangtze estuary. The traditional field survey in local areas is, however, not sufficient for reliable forecast of the development trend of geomorphological pattern of the Yangtze estuary in several decades. Salt marshes in the Yangtze estuary are located in the key areas influenced markedly by the land-ocean interactions, and the development of sedimentary sequences therein is impacted directly by fluxes of both water and sediment from the drainage basin of the Yangtze River. Vertical profiles of salt marshes will be selected in two transactions at the directions with the greatest progradation rate in the east of Chongming island and the east of Nanhui coast respectively, for the researches in this project. Based on data of sedimentology, elemental and isotopic geochemistry, environmental magnetism and isotopic dating, the development patterns of different vertical profiles in the past 100 years will be established for the salt marshes. The responses of the development of salt marsh sedimentary sequences to the variations in fluxes of both water and sediment from the drainage basin of the Yangtze River will be investigated, to provide historical analogues for evaluation of the development trend of the salt marshes in the Yangtze estuary under the future scenarios of fluxes of both water and sediment to the sea. This project will also provide a typical case of one Chinese estuary with high turbidity for the global investigations on the responses of the estuaries to the drainage basins of rivers flowing into the sea.

河口海岸发育对流域环境变化与人类活动的响应，是近期多个国际科学组织（如，IGBP、LOICZ、IAG）的核心研究内容之一。三峡大坝与南水北调工程的实施对未来长江口地貌格局的影响，已成为河口海岸相关学科迫切需要回答的焦点科学问题。现场地貌与水文连续观测是了解长江口近期地貌动态的有效选择；但是，对于长江口未来几十年地貌态势的可靠预测，仅依据传统的局部现场观测是不够的。长江口盐沼处于陆海相互作用关键地带，其沉积层序发育受到流域来水来沙的直接影响。本项目选择长江口崇明岛东部与南汇东滩近百年来淤积速率最大方向上的若干盐沼剖面，综合沉积学、元素与同位素地球化学、环境磁学及同位素测年资料，建立近百年来本区盐沼沉积剖面的发育模式，研究盐沼沉积层序发育对流域入海水沙通量变化的响应，为预测未来入海水沙情景下长江口盐沼演变趋势提供历史相似型。本项目亦可为全球入海河口对流域的响应研究提供中国高浊度河口的典型实例。

流域水文过程对河口地貌演化的制约，是流域到河口整合研究的核心内容之一。本项目通过研究作为河口典型地貌单元的盐沼的发育过程，揭示近百年来长江口盐沼沉积记录的物源信息，进而了解流域水文过程的变化。本项目利用筑造年代确切的若干海堤作为历史岸线的可靠指示，综合盐沼柱样的沉积特征，确定了近百年来长江口盐沼发育的时空特征与趋势。从横向上岸线向海迁移的速度，以及纵向上盐沼沉积记录的盐沼发育特征入手，参照同期长江入海水沙通量的变化，研究近百年以来流域来水来沙变化对长江口盐沼发育的影响。.近六十年来崇明岛东部盐沼淤涨呈先快、后慢、再快的趋势，这与同期长江年入海泥沙通量的变化趋势基本一致，表明近六十年来崇明岛东部盐沼的淤涨主要受长江入海泥沙的制约。沉积物有机碳稳定同位素结果表明，近六十年来崇明岛东部盐沼有机质主要来自长江径流携带的悬移质。近六十年来崇明岛东部盐沼柱样沉积物呈变细趋势，这与同期长江入海泥沙粒径变化的趋势一致。因此，长江入海泥沙是崇明岛东部盐沼发育的物质来源。.南汇地区近百年来的盐沼沉积层段，有机碳δ13C值介于-26.4‰ ~ -22.4‰，与长江口悬浮颗粒物的δ13C值一致。沉积物POC含量、TN含量与有机碳δ13C值表明，盐沼有机质组成受到流域水沙变化与人类围垦等的影响。南汇地区钻孔的中潮坪层段沉积物自陆向海变细，这与长江口悬沙中值粒径上世纪80年代后期呈变细趋势一致。因此，近百年以来南汇地区盐沼沉积物组成受到流域来沙的显著制约。近六十年来崇明岛东部不同钻孔潮坪沉积层段，泥质纹层的平均厚度逐渐增加，砂质纹层的平均厚度逐渐减小。这与上世纪60年代以来，长江入海泥沙逐渐变细的趋势一致，说明近六十年来崇明岛东部盐沼沉积层序的发育受长江入海泥沙的控制。本项目取得的研究结果，为科学评估长江口盐沼湿地在未来流域自然因素与人类活动双重影响下的演变趋势提供了科学依据。